Control circuits for series connected semiconductors



United States Patent CONTROL CIRCUITS FOR SERIES CONNECTED SEMICONDUCTORS Georg Sichling, Erlangen, Germany, assignor to Siemens- Schuckertwerke Aktiengesellschaft, Erlangen, Germany, a corporation of Germany Filed Oct. 31, 1955, Ser. No. 543,997

Claims priority, application Germany Nov. 17, 1954 11 'Claims. (Cl. 307-885) My invention relates generally to semi-conductors and has reference in particular to electric control circuits for series connected controllable semi-conductors which are used to control loads at voltages higher than the rated blocking voltage of a semi-conductor.

Where it is desirable to control loads at voltages which are higher than the rated or blocking voltage of a semiconductor, several controllable semi-conductors can be connected in series, but it is essential that they be controlled simultaneously. Common control of several series connected semi-conductors, such as transistors, is very difficult, particularly in cases where the transistors are connected in a circuit with a grounded emitter. According to the present invention the control electrodes of a plurality of series connected semi-conductors are connected to a common control circuit in such manner that the potentials of the control electrodes are separated.

Generally stated, it is an object of my invention to provide in a simple and efiective manner for simultaneously controlling a plurality of semi-conductors which are connected in series circuit relation.

More specifically, it is an object of my invention to provide for controlling a load which operates at a voltage higher than the rated or blocking voltage of a single semiconductor, by means of a plurality of series connected semi-conductors which are controlled by means of potentials which are varied simultaneously but which are separated from each other.

Another object of my invention is to provide for separating the control or base electrodes of a plurality of series connected transistors by resistances having ohmic values approximately equal to the blocking resistances of the respective transistors.

Yet another object of my invention is to provide in a control circuit for several series connected semi-cond-uctors, for connecting resistances in parallel with different ones of the semi-conductors for the purpose of adjusting the currents through the semi-conductors to values which will limit the voltages across the semi-conductors to permissible values.

It is also an object of my invention to provide in a control system for using a plurality of series connected transistors to control a load at a relatively high voltage and for supplying control voltages to the control electrodes thereof from separate windings on a transformer having a common primary winding to which is applied a variable control voltage.

It is an important object of my invention to provide in a control system for transistors, for using either a saw-tooth shaped primary current or a rectangular waveform voltage in a control transformer for applying control voltages to the control electrodes of the transistors, so as to operate them in the middle portion of the power curve characteristic thereof, as little as possible.

Other objects will, in part, be obvious and will, in part, be described hereinafter.

In practicing my invention according to one of its embodiments, several transistors are connected in series circuit relation so as to control a load operating at a voltage which is higher than the rated voltage of a single transistor. Simultaneous switching of all of the transistors is effected by connecting between the base and emitter of each transistor a separate secondary winding of a control transformer having a common primary winding. The primary winding is energized from an alternating current source through a pair of transistors which selectively control alternate half cycles in response to an applied direct current control voltage.

For a more complete understanding of the nature and scope of my invention, reference may be made to the following detailed description which may be read in connection with the accompanying drawing, in which:

Figure 1 is a diagrammatic view of a control circuit embodying the invention in one of its forms;

Fig. 2 is a diagrammatic view of a control circuit embodying the invention in a different form;

Figs. 3 and 4 show curves illustrating typical current and voltage collector characteristics of the transistors; and

Fig. 5 shows a typical saw-tooth shaped current and a rectangular Waveform voltage which may be utilized in connection with one embodiment of the invention.

Referring to Fig. l, a load 2 is shown being supplied from a voltage source 1 which has a voltage higher than the blocking or rated operating voltage of a single transistor. Transistors 3, 4, 5 and 6 are shown connected in series circuit relation with the load 2 and battery 1 for purposes of illustration. Each of the transistors 3, 4, 5 and 6 includes an emitter electrode 3e 4e, 5e and 6e, respectively, and a collector electrode 3c, 40, 5c and 60, respectively. The number of such series connected transistors is obtained in a known manner from the relation of the open circuit voltage of the voltage source 1 and the rated blocking voltages of the respective transistors. Van'- able resistors 7, 8, 9 and 10 are connected in parallel circuit relation with the transistors so that their adjustable taps h, g, f and e, respectively, are connected to the base electrodes 3b, 4b, 5b and 6b. Variable voltage sources 11 and 12 are connected in series With the resistances 7 through 10 having a polarity opposite to that of the voltage source 1. The resistances 7 through 10 are preferably of ohmic values which are approximately equal to the blocking resistances of their respective transistors.

By means of the above-mentioned variable voltage sources 11 and 12 the potentials of the points e through h are adjusted in such a manner that the net potentials between the points a-e, b-f, c--g and d-h are zero. Thus, no bias voltage is applied to the base electrodes, and the transistors will be in a blocking condition. When the voltages 11 and 12 are either reduced or the voltage sources short circuited, the potential of the whole resistance series increases and the potentials at the points e through h may be made suificiently negative with respect to the points a through d to render the transistors 3 through 6 conductive. This reduces to a minimum the effective resistance of the transistors, so that current flowing through the transistors and the load 2 produces only a small voltage drop on the transistors and substantially the entire voltage of the source 1 is applied to the load 2. The resistances 7 through 10 and the taps e through It thereof may be adjusted in such a manner that despite the small voltage remaining across the series resistances the electrodes 3a through 6a are modulated to maintain the transistors conductive.

In order to obtain a more nearly uniform potential distribution on the transistors 3 through 6, an additional series circuit of resistors 17 through 20 is connected in parallel with the transistors so as to maintain the potentials of the points b through d uniform during the blocking period of the transistors. In this event, simultaneous control of the transistors 3 through 6 can be obtained by increasing the potential of the series resistances 17 through 20 by means of additional series connected voltage sources 21 and 22 which are cumulative with respect to the source 1, while the potentials of the resistance series 7 through 10 are kept constant. Control can also be obtained by moving the taps e through It to one end of the respective resistance and increasing the voltages of the sources 11 and 12.

The above described operation necessitates substantially identical transistors, that is transistors having the same voltage, transit resistance, current amplifying coefficient, control voltage increase value, and time constant for firing and extinguishing. In case the respective values should differ from each other, particularly when the values of the blocking voltages of the different transistors should vary, the system can be balanced by appropriate values of resistors 7 through 10 and 17 through 20, to obtain a more equal distribution of voltages among the transistors. If the transit or through resistances of the current transistors differ at equal voltages, balance can be restored by introducing resistances 23 through 26 in circuit with the base electrodes 311 through 6b. Different values of gain or control voltage increase among the transistors can be balanced by introducing additional voltage sources having appropriate values such as the sources 27 through 30, to compensate for such differences.

Two typical cases for the characteristics (collector current i and collector voltage u for a short-circuited transistor between emitter and base are shown in Figs. 3 and 4. In Fig. 3 the curve 51 represents the characteristic of one transistor, for example, the transistor 3, and

52 designates the characteristic of another transistor, for example, the transistor 4. When both transistors are series-connected, a bias voltage U will appear on the transistor 3 at a certain current I A voltage U will appear on the other transistor 4. The sum of the voltages U and U is, of course, equal to that portion of the open circuit voltage of the voltage source across these transistors. The voltage U could be higher with regard to the stability of the transistor 3 but this would induce a current 1 which would produce in the transistor 4, having the characteristic 52, an excessive voltage drop. If an appropriately valued resistance 17' is connected across the transistor 4 having the characteristic 52, the currents of both transistors can be adjusted so that the voltage on each transistor is just below the permissible maximum value.

In Fig. 4 the characteristic of one transistor, for example, the transistor 5, is designated by the curve 61, and that of another transistor, the transistor '6, for example, the curve 62. It will be observed in this case the transistor characteristics have different saturation characteristics. The voltage distribution on the transistors will be even more critical than with the characteristics shown in Fig. 3. A current J flows in the transistor with the characteristic curve 62 with the maximum permissible voltage U A corresponding very small voltage U appears on the transistor 5 having the characteristic curve 61. This transistor will, therefore, receive a nearly negligible voltage. In this case the balance can be restored by means of a resistance 20' which is connected in parallel with the transistor 6 having the characteristic curve 62 so that a current J will flow through the transistor with the characteristic curve 61 at a corresponding voltage U' It will thus be recognized that by appropriately valued resistances connected in parallel with the transistors having different voltage characteristics each transistor can be adjusted to operate at the maximum operating voltage. The resistances 17 through 20 may also be used for balancing the operating voltages of the respective transistors.

Referring to Fig. 2, it will be seen that the base elec' trodes of the transistors 3 through 6 are connected to the secondary windings 32 through 35 of a control transformer 31 so that they are completely insulated from eachother. In this instance, the ohmic value of the resistance connected between the base electrodes approaches infinity. Rectifier bridge circuits 41 through 44 are used to connect the windings 32 to 35 to the base electrodes 3b through 61) of the transistors, so as to apply positive voltages to the base electrodes for the purpose of rendering the transistors non-conducting. The primary winding 31 of the transformer is connected to an alternating current source 45 and two parallel connected transistors 36 and 37 connected in parallel with each other are connected in series with the source and the primary winding. The transistor 36 may be of the n-p-n type having an emitter 36a and a collector 360, while the transistor 37 may be of the p-n-p type having an emitter 372 and a collector 37c. Rectifiers 38 and 39 are connected in circuit with the transistors 36 and 37 in the opposite directions for controlling opposite half cycles of the alternating current. The base electrodes 36b and 37b are connected to a variable direct current source 46, so as to provide for impressing a variable direct current voltage on the base electrodes for controlling the alternating current in the primary winding 31'.

By changing the direct current voltage applied to the base electrodes, the primary current of the transformer 31, and thus the secondary current of the transformer is changed. This controls the voltages applied to the base electrodes of the transistors 3 through 6. This method of transformation of direct current efiectively into an alternating current which is then rectified, enables a simultaneous control of series connected transistors with complete separation of potentials between the individual control electrodes. Resistors 17 through 20 may be connected in parallel with the semi-conductors 3 through 6 to obtain a more uniform voltage distribution, if desired.

A continuous control of the transistors 3 through 6 is possible with the arrangement of Fig. 2 by varying the amplitude of the primary alternating voltage of the transformer 31. This continuous control is possible only so long as the resistance characteristic of the load does not cross the power characteristic hyperbola curve of the transistor, that is as long as a permissible output of the transistor is not exceeded at any point in the control range.

Sometimes it is necessary to operate a transistor at a higher power than would be permissible With regard to heating of the transistor. In this instance, different measures are possible to permit a power control for transistor outputs higher than that normally permissible. These measures are based on the fact that the power loss of a semi-conductor, because of the hyperbolic shape of the characteristic is substantially smaller in the unmodulated portion and the fully modulated or portion, than for other intermediate values, particularly in the middle portion of the modulated range. It is, therefore, suggested to pass this middle range as quickly as possible so that the semi-conductor will be operating mostly in either a blocking condition or in a conducting condition and the power losses during the intermediate condition will occur for only a very short time so that excessive heating and possible damage of the semi-conductor are eliminated.

The transistors 3 to 6 in Fig. 2 of the drawing may be controlled in this manner by applying to the primary of the transformer 31 either a rectangular waveform voltage or a saw-tooth shaped current according to the type of converter used. Fig. 5 shows a saw-tooth shaped primary current applied to the transformer. As long as the difierential quotient air in Fig. 5 remains positive, that is, in the range (A-B), a negative voltage DE, as shown in Fig. 6, will be induced in the secondary which will be able to render the transistors 3 through 6 conductive. But, when the quotient ii dt is negative in the range BC, the secondary voltage will increase to a corresponding positive value FG, so that the transistors will block. If the current applied to the primary of the transformer is changed to such an extent that it attains the shape AB'C shown by the dotted line in Fig. 5, then the conductive period D'E' of the transistor will be reduced and the blocking period FG will be longer. It thus becomes possible to modulate the transistor for operation at a level above the normally permissible value Without overloading the transistor. A further advantage of this arrangement is that the rectifiers 41 through 44 of Fig. 2 may be eliminated.

Since certain changes may be made in the abovedescribed construction, and difierent embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the above'description and shown in the accompanying drawing shall be considered as illustrative and not in a limiting sense.

I claim as my invention:

1. In an on-ofi switching circuit, a plurality of seriesconnected controllable semi-conductors connecting a load to a direct current source having a voltage higher than the blocking voltage of each of said plurality of semiconductors, each of said semi-conductors having a control electrode, circuit means applying only simultaneously variable control voltages to the electrodes and including a voltage divider comprising a plurality of resistors series connected with the source and disposed in parallel with the series connected semi-conductors, said resistors connected to the control electrodes to maintain the semiconductors in similar states of control as the controlled state varies in response to application of said control voltages.

2. Control apparatus for switching a load comprising, a plurality of semi-conductors connected in series with the load and a direct current source having a voltage higher than the blocking voltage of each of said plurality of semi-conductors, each of said semi-conductors having a control electrode, circuit means connecting the control electrodes to the source and including a plurality of resistors connected in series with the source, one resistor for each semi-conductor and said resistors each having an ohmic value equal to the blocking resistance of the respective semi-conductor, each resistor connected in circuit relationship with the control electrode of the corresponding semi-conductor.

3. In a switching control circuit for a load, a plurality of series-related semi-conductors connecting the load to a source having a voltage higher than the blocking voltage of each of said plurality of semi-conductors, said semi-conductors each having a control electrode, a plurality of resistors connected in parallel with the semiconductors, circuit means connecting a different one of said resistors to each control electrode, and variable voltage means connected in series with said resistors in opposition with said source.

4. Control means for switching a load on-oiI comprising, a plurality of series-connected semi-conductors connecting the load to a direct current source providing a voltage higher than the rated blocking voltage of each of said plurality of semi-conductors, each of said semiconductors having a control electrode, circuit means applying simultaneously variable control voltages to the electrodes, and fixed value impedance means connected in series with the source and in parallel with the semiconductors to provide a more uniform voltage distribution among the semi-conductors.

5. In an on-ofi control circuit for a load, a plurality of serially-connected semi-conductors connecting the load to a source, said semi-conductors each having a base control electrode, a control transformer having a common primary winding and a plurality of secondary windings, rectifier means connecting each secondary winding to a different base control electrode, and means energizing the 6 primary winding with an abruptly changing signal for effecting switching of the load.

6. An on-off switch circuit for a load comprising, a plurality of series-connected semi-conductors connecting the load to a direct current source, each of said semi-conductors having a base control electrode, a control transformer having a common primary winding and a secondary winding individual to each semi-conductor, rectifier means connecting each secondary winding to a different base control electrode, and means including a pair of oppositely disposed controllable semi-conductors connecting the primary Winding to an alternating current source flor applying an on-ofi. signal to the base control electrodes.

7. In switching apparatus for a load, a plurality of series-connected semi-conductors connecting the load to a direct current source, each of said semi-conductors having a control electrode, means including -a control transformer having a common primary winding and a plurality of secondary windings connected by rectifier means each to a different control electrode, and means selectively energizing the primary winding including an alternating current source having a rectangular waveform.

8. Control apparatus for a switching load comprising, a plurality of series-connected semi-conductors connecting the load to a direct current source, each of said semiconductors having a control electrode, a control trans former having a common primary winding and a plurality of secondary windings connected individually to the different control electrodes by rectifier means, and means for applying an alternating current to the primary wind ing having an abruptly changing waveform.

9. In an electrical system, a source of voltage, a load, coupling means including a current path for coupling the source to the load, said current path including in series a plurality of impedance elements of relatively fixed value, and a plurality of semi-conductor circuit devices each shunting a separate one of the impedance elements, said circuit devices under a certain first condition of said electrical system simultaneously offering a first impedance to current flowing therethrough from the source, said circuit devices under a certain second condition of the electrical system simultaneously offering an impedance substantially greater than the first impedance to current flowing therethrough from the source, certain of the circuit devices under said second condition of the system being subjected to undesirable performance for voltages thereacross of an order appearing across a plurality of the impedance elements in series, each of the impedance elements having a value selected to maintain the voltage across the associated circuit device below a value causing said undesirable performance.

10. In an electrical system, a source of unidirectional voltage, a current path connected for energization from said source, a plurality of semi-conductor electroresponsive valve devices each having at least three electrodes, a pair of electrodes of each device being connected in said current path, means connected between a third electrode and one electrode of the pair of electrodes of each of said devices for normally maintaining said devices in a substantially non-conducting condition, and operable to provide a conducting condition between the pair of electrodes, said source providing at least a portion of a voltage which is blocked by said devices when in a nonconducting condition, and a separate resistor of relatively fixed value connected across the pair of electrodes of each device, each of the resistor elements having a value selected to maintain the voltage across the associated circuit device below a value causing undesirable performance.

11. Control means for switching a load on-ofi comprising, a plurality of series-connected semi-conductors connecting the load to a direct current source providing a voltage higher than the rated blocking voltage of each of said plurality of semi-conductors, each of said semiconductors having a control electrode, circuit means applying simultaneously variable control voltages to the electrodes, impedance means comprising a plurality .of fixed value impedance elements, means individually connecting said impedance elements in parallel with certain of said semi-conductors and all of said impedance means in series with said source, each of said certain semi-conductors and its parallelly connected impedance means comprising an impedance unit, the magnitude of each said impedance means being so related to the corresponding semi-conductor that each impedance unit will have a 10 voltage drop thereacross which does not exceed a value causing undesirable performance to provide a more uniform voltage distribution among the semi-conductors.

1,985,923 Gutmann Jan. 1, 1935 ,8 Eberhard ,Dec.'5, 1950 Barney Feb. 12, 1952 Darlington Dec. 22, 1953 Shockley Jan. 19, 1954 Sziklai et a1. Jan. 17, 1956 Doremus et a1 Oct. 29, 1957 Houck Jan. 7, 1958 Linvill et al. Apr. 15, 1958 FOREIGN PATENTS France "June 9, 1954 OTHER REFERENCES Anderson: Transistors in Switching Circuits, pages 1541-1558 of November 1952 IRE, vol. 40, No. 11. 

